Sheet Metal BasicsForum for those beginning sheet metal layout and other related fields. Let the industry help answer your questions. There's plenty of experienced help in our shop wanting to help you.

So I'm doing a furnace change over and really don't like the size of the return air shoe tying into the furnace, I'd like to increase both it's width and height, the R/A drop is 8x21.5 and I would like to end up with 20x25, I would just put a 2 inch square throat on it and that leaves me with 12 inches of straight on my heel, my problem comes with laying out the drop cheek portion. I've read the guide on the drop cheek on this site and was very impressed, but I just don't know if I find just the centre of the radius portion of my heel or the radius plus the straight. Also unsure of where to stoke my arcs from, I'm sure I start on the 8X21.5 side but from there I kind if get lost, been awhile since I built my own fittings but with the prices of custom metal going crazy a guy has too, sorry for the wall of text for my first post but I thought I would come in with a bang!

In my own house I didn't worry about a radius on the return drop, just spread it out to allow plenty of volume. Can you insert a transition in the drop to, say, 8 x 24 or more, and then tie in the 20 x 25 from the heater on the horizontal? If there's room that's (the horizontal) also a good place for a media-type air filter.

My wife just asked about when the heat will be on. I said, come on, it's only September.....I got THE LOOK. Hate to think about buying oil!

So I'm doing a furnace change over and really don't like the size of the return air shoe tying into the furnace, I'd like to increase both it's width and height, the R/A drop is 8x21.5 and I would like to end up with 20x25, I would just put a 2 inch square throat on it and that leaves me with 12 inches of straight on my heel, my problem comes with laying out the drop cheek portion. I've read the guide on the drop cheek on this site and was very impressed, but I just don't know if I find just the centre of the radius portion of my heel or the radius plus the straight. Also unsure of where to stoke my arcs from, I'm sure I start on the 8X21.5 side but from there I kind if get lost, been awhile since I built my own fittings but with the prices of custom metal going crazy a guy has too, sorry for the wall of text for my first post but I thought I would come in with a bang!

No time this morning but let me try to help tonight.
I have a dvd that might help if you too.

looking at you picture.. how necesary is it to have a radius heel on the return.. you actually want the air to slow right down at this point and deposit the air particles evenly in the filter..

if you do use a curved heel it tends to speed up the air by alowing it to smoothly flow along the radius and collect the particles towards the bottom of the filter..

if the air slows and even has a little turbulance at the elbow it spreads out and slows.. slower air across the filters equal better airfiltration..

A heel radius is not necessary, but a radius throat is absolutely necessary.
Every elbow we fabricate has a radius throat no matter how small.
This allows the air to 'hit' the top of the filter.
Turning vanes are even better.
I have an equivalent length chart around here somewhere....